* Fam Zheng <famz@xxxxxxxxxx> wrote: > On Sun, 02/15 15:00, Jonathan Corbet wrote: > > On Fri, 13 Feb 2015 17:03:56 +0800 > > Fam Zheng <famz@xxxxxxxxxx> wrote: > > > > > SYNOPSIS > > > > > > #include <sys/epoll.h> > > > > > > int epoll_pwait1(int epfd, int flags, > > > struct epoll_event *events, > > > int maxevents, > > > struct epoll_wait_params *params); > > > > Quick, possibly dumb question: might it make sense to also pass in > > sizeof(struct epoll_wait_params)? That way, when somebody wants to add > > another parameter in the future, the kernel can tell which version is in > > use and they won't have to do an epoll_pwait2()? > > > > Flags can be used for that, if the change is not > radically different. Passing in size is generally better than flags, because that way an extension of the ABI (new field[s]) automatically signals towards the kernel what to do with old binaries - while extending the functionality of new binaries, without sacrificing functionality. With flags you are either limited to the same structure size - or have to decode a 'size' value from the flags value - which is fragile (and in which case a real 'size' parameter is better). in the perf ABI we use something like that: there's a perf_attr.size parameter that iterates the ABI forward, while still being binary compatible with older software. If old binaries pass in a smaller structure to a newer kernel then the kernel pads the new fields with zero by default - that way the kernel internals are never burdened with compatibility details and data format versions. If new user-space passes in a large structure than the kernel can handle then the kernel returns an error - this way user-space can transparently support conditional features and fallback logic. It works really well, we've done literally a hundred perf ABI extensions this way in the last 4+ years, in a pretty natural fashion, without littering the kernel (or user-space) with version legacies and without breaking existing perf tooling. Other syscall ABIs already get painful when trying to handle 2-3 data structure versions, so people either give up, or add flags kludges or go to new syscall entries: which is painful in its own fashion and adds unnecessary latency to feature introduction as well. Thanks, Ingo -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
